The origin and evolution of Tyrannosauroidea [1] is one of the interesting puzzles in theropod phylogeny (the only one more fascination (with all due apologies to Tom Holtz) [2]. Inclusion of tyrannosaurids is a no-brainer, but the origin of tyrannosaurs has found itself in strange company, including rooted close to the origin of Maniraptora (oviraptorosaurs, dromaeosaurs, birds, and troodontids, and a few other odds and ends) [3], as the sister taxa to ornithomosaurs [4], or as the most basal lineage of Coelurosauria [2], to name just a few.

In the last few years, some old and some new taxa have been described and their evolutionary placement among or around tyrannosaurs has provided a fascinating perspective on what it might mean to be a dinosaur tyrant. Only the big tyrannosaurids seem to be really “tyrant-like” at all, with their giant, robust skulls, large body size, short arms, reduced manual digits, and so on. Basal tyrannosauroids don’t seem to have any of these features, but have a few other finer details. They seem to fall into three grades of tyrannosauroid: the tyrants, described as above as large-bodied, large-headed, small-armed, and few-fingered; the other two groups seem to be grades in the explicit sense, as they do not form explicitly monophyletic groups in and of themselves.

This led me to coin the term “despot” for the first group, which includes the “psuedo”-tyrants like Guanlongwucaii. Here, “despot” would simply be a “minor” tyrant, and certainly not on the order a truly “ruling” lizard like the Tyrannosauridae. Some of them even appear to form a smaller clade within the larger Tyrannosauroidea, such as Proceratosauridae [5]

“Despots” and “peons.” Skulls are scaled to the same base length (posterior margin of the distal quadrate condyles to the anterior margin of the alveolar rim of the premaxilla), or an equivalent measure when material is lacking. All scale bars equal 5 cm.

Above is a showcase of the cranial material of various tyrannosaur-like dinosaurs, some of which are labeled despots, and some as peons.

I am not the first to implicate a grouping of basal coelurosaurs, although I did so several years ago [15,16] on the basis of the foreshortened and medially deflected premaxilla, an anterior process on the postorbital projecting into the orbit, etc.; Greg Paul [18] implicated a relationship between Ornitholestes hermannii and Proceratosaurus bradleyi on several features including the apparent nasal crest (absent in Ornitholestes [9]), shape of the squamosal and the nature of its contact with the quadratojugal, and the radial arrangement of the mesial dentition of the dentary.

As time advances, some of these theropods seem less and less like “tyrants” than others, and this has suggested that some may be non-despots simply because they fall outside a basic “tyrant”-like morphology. Some of them included the long-armed, vaguely compsognathid-like Dilong imperator [10], and may have also included Sinosauropteryx prima (or rather, a specimen referred to it [19]), and even Ornitholestes hermanni [9] even though it has autapomorphically shortened its skull distinctly.

Previous workers have also included additional taxa into Tyrannosauroidea that are not shown above due to the lack of distinct cranial material, although some braincases are known. They include Iliosuchus medullaris [13], although [20] disagreed and indicate it may be more basal than Coelurosauria; Itemirus medullaris [21], although again someone [22] disagreed, this time placing it among the Dromaeosauridae; Bagaraatan ostromi [23], originally described as a generalized and indeterminate coelurosaur, before being placed into the Tyrannosauroidea by [2] and [25]; Aviatyrannis jurassica [26], which at first drew similarities to Iliosuchus incognitus and then to Stokesosaurus clevelandi, and is placed with the latter [2]; and Xiongguanlong baimoensis [27], which stresses a Guanlong-like anatomy with a vairly dervied-looking skull.

Dryptosaurus aquilunguis, originally named Laelaps aquilunguis [27], but which was preoccupied, is a somewhat shaky tyrannosauroid that may be close to the root of Tyrannosauridae [28], seems close to Appalachiosaurus montgomeriensis. It may thus be a despot, along with Appalachiosaurus, or a true tyrannosaurid if the latter is associated with albertosaurines as also suggested by [28].

Tanycolagreus has features in common with tyrannosauroids, at least, while the others have slightly less features, or have features in common with tyrannosauroids that are plesiomorphic for them (which doesn’t necessarily disqualify them). Carr and Williamson are currently working on a broader tyrannosauroid-based analysis, in which I am certain many of these taxa’s relationship, Sinosauropteryx and comsognathids (also implied to be close) can be resolved to some degree.

Very nice post! Non-tyrannosaurid tyrannosauroids are a very fascinating but poorly understood group. Hope they will find new COMPLETE remains as soon as possible! What bout Embasaurus, AMNH 2906 and the undescribed tyrannosauroid from Bissekty Formation?